Conventionally, in image forming apparatuses such as a multifunction color copying apparatus (an MFP), types of media on which color image and the like are printed increase. Even when printing media are limited to paper, various types of paper having different thicknesses and the like are used.
Such types of paper are usually distinguished by basis weight (unit: g/m2) indicating weight per a fixed area. For example, papers in groups of basis weights 64 to 105, 106 to 163, 164 to 209, 210 to 256, and 257 to 300 are called plain paper, thick paper 1, thick paper 2, thick paper 3, and thick paper 4. These basis weights are usually written on packages of sheets. A user selects one of the groups of the written basis weights, whereby a printing condition corresponding to a type of paper belonging to the selected group is automatically set. In general, it is desirable to change a printing condition according to basis weight. However, basis weight may be unknown. In such a case, it is difficult to easily detect basis weight. It is possible to calculate basis weight from the density and thickness of a sheet. Although it is difficult to measure the density of a sheet, the measurement of paper thickness is relatively easy compared with the measurement of density. Therefore, there are known devices that detect paper thickness in order to learn a paper type and the like.
One of these paper thickness detecting devices includes a pair of conveying rollers that convey a sheet. The paper thickness detecting device measures a displacement amount of the conveying rollers when the sheet is passed between the conveying rollers and detects paper thickness from this displacement amount. In other words, the conveying rollers are used also as paper thickness detecting rollers.
In another one of the paper thickness detecting devices, a roller exclusively used for paper thickness detection is provided separately from the conveying rollers, a counter plate is provided to be opposed to this roller. The paper thickness detecting device measures an amount of displacement of the paper thickness detecting roller by a sheet entering between the counter plate and the paper thickness detecting roller and detects paper thickness.
Still another one of the paper thickness detecting devices includes, for example, as disclosed in JP-A-2003-237982, a driving roller instead of the counter plate and rotates the driving roller in synchronization with the conveyance of a sheet.
Such paper thickness detecting devices in the past detect paper thickness in any case. A paper type is estimated from the paper thickness. Therefore, usually, this paper thickness detection is performed, density is assumed, a paper type is estimated, and a printing condition, for example, fixing temperature of a toner image developed with a toner is changed according to the thickness of a sheet detected by the paper thickness detection. Consequently, in most cases, satisfactory printing can be performed.
However, as described above, even if paper thickness is fixed, when the density of the sheet is outside an assumed range of values, it is likely that basis weight changes and an optimum printing condition changes. In such a case, a sufficiently satisfactory printed image may not be obtained.